This relates to the concept of and means for performing Acoustic Emission Linear Pulse Holography, which combines the advantages of linear holographic imaging and acoustic emission testing into a single non-destructive inspection system. This unique system produces a chronological, linear holographic image of a flaw by utilizing the acoustic energy emitted during crack growth.
Conventional linear holographic imaging uses an ultrasonic transducer to transmit energy into the volume being imaged. When the crack or defect reflects that energy, the crack acts as a new source of acoustic waves. To formulate an image of that source, a receiving transducer is scanned over the volume of interest. The phase of the received signals is then measured at successive points on the scan. The phase information can be reconstructed to formulate an image of the defect.
Conventional acoustic emission testing methods utilize the energy given off by a crack as it grows in a monitoring system to detect and locate it. This is done by measuring the time of arrival of an elastic wave at a group of sensors (usually 2 to 4) and then computing the crack location by using triangulation or other computational techniques. Typically the location accuracy is no better than one wall thickness. No direct measure of crack size can be obtained by such systems.
The innovation disclosed in this disclosure is the concept of utilizing the crack-generated acoustic emission energy to generate a chronological series of images of a growing crack by applying linear, pulse holographic processing to the acoustic emission data. The process is implemented by placing on a structure an array of piezoelectric sensors (typically 16 or 32 of them) near the defect location. A reference sensor is placed between the defect and the array.
The short bursts of acoustic emission generated by stress, etc, propagate through the medium to the discrete-element receiver array. The reference timing sensor that is positioned between the array and the inspection zone initiates time-of-flight measurements to each sensor in the array. The acoustic wave signals are sampled at each position across the array and time-of-flight data are measured at each position. The aperture data (i.e., series of measurements) are then transferred to a computer for reconstruction of a timed series of linear holographic images. Computer reconstruction of the images can be accomplished using a one dimensional FFT algorithm. Images can be displayed on the computer terminal graphics console. All image data can be stored on digital tape cartridges to allow generation of a chronological history of crack growth with respect to the material depth, etc.
The general concept of using short ultrasonic pulses to generate synthetic frequency holograms was first presented with respect to underground mining applications and later applied to imaging of underground pipes. However, in these conventional techniques a pulse of ultrasound energy is transmitted into the medium being inspected and then the time until the arrival of the returned echo is measured. Acoustic emission Linear Pulse Holography differs from conventional linear pulse holography in that the acoustic energy emitted by the defect itself is used to generate the time-of-flight information to a receiving array of sensors.
Conventional acoustic emission monitoring performs source location by measuring the time of arrival of an acoustic emission wavefront at 2 to 4 sensors. The time information is used indirectly to estimate the location of the source. This method only locates the general area from which the sound wave originated. It does not give image-type information about the shape and growth of the actual crack front. Hence, acoustic emission Linear Pulse Holography differs from conventional acoustic emission monitoring in that (1) linear pulse holographic techniques are used to locate the source, rather than for time triangulation computations, and (2) acoustic emission Linear Pulse Holography has sufficient resolution to continuously image the changing shape of a crack front or defect boundary.